Abstract

Earth’s energy imbalance (EEI) determines whether the planet experiences a net gain or loss of energy. The ongoing rise in atmospheric greenhouse-gas concentrations causes a positive EEI , which ultimately drives global warming. Today, most of this excess heat is taken up by the surface ocean. On millennial to orbital timescales, by contrast, energy partitions between two considerably larger but slower-responding reservoirs: the global (deep, intermediate, and surface) ocean and the latent heat involved in waxing and waning land ice. Ocean heat content (OHC) and global sea level, which mirrors land ice volume, are therefore key metrics to assess the global energy balance during the Quaternary.

Past OHC can be reconstructed by analyzing noble-gas ratios in polar ice cores. This method relies on two properties of noble gases: firstly, noble gases have temperature-dependent and species-specific solubility in seawater; and secondly, noble gases are chemically inert, such that their total inventory is conserved in the coupled ocean–atmosphere system. Ice cores contain ancient air, from which past atmospheric noble-gas ratios can be reconstructed. Using the above-mentioned properties of noble gases, past mean ocean temperature and OHC can then be derived from these atmospheric noble-gas ratios.

Here, I present a new OHC dataset, which spans over an entire glacial cycle (MIS 9–7) and covers the last four glacial terminations in millennial resolution, substantially extending the existing OHC record. By combining this new OHC record with past sea-level reconstructions, we obtain an EEI record across an entire glacial cycle. This EEI record not only features orbital-scale variability in response to the albedo and greenhouse-gas forcing but also exhibits strong millennial power. These millennial-scale EEI changes, which are mirrored in OHC , have consequences for the interpretation of past sea-level, atmospheric CO2 , and climate.